Justeaze utilized her own body to become the magic circuit of the Grail.At the time, Zouken admired her deeply, and was devastated to see her sacrifice.
Disappointed with the lack of sorceric potential in Shinji and Byakuya, Shinji's father, Zouken blames Kariya for allowing the Matou's magical lineage decline since Kariya is the first in many generations to be naturally capable of sorcery.
However, Kariya formulates a deal with Zouken where should Kariya obtain the Holy Grail, Zouken must allow Sakura to return to her family.
He wanted to eliminate and cure all hatred and suffering.
He felt grief upon finding that "paradise" did not exist in the world and its creation was impossible with a human body, and he instead wished to go to a place where it could be accomplished.
Zouken considers Shinji to be a disgrace of a sorcerer, and believes Sakura may have more potential than her brother.